“From the outside it may look like their lives are going well, but they have a strong sense of being unfulfilled. They may feel they have underachieved – even if they are successful students or doctors.

“Becoming involved in an extremist cause gives their life a sense of meaning they didn’t have before, and can make them feel they are doing something really important, that is making a big difference.”

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“If young men watch this type of stuff their attitudes shift. They gravitate even more strongly to the extremist groups they identify with and they become increasingly hostile towards anyone they think is attacking or disrespecting that group.

“They also show increased willingness to carry out suicide attacks to support these groups – even if they previously devoted themselves to saving lives as doctors like Hassane and Abdullah did.”

Prof Silke says studies have shown that people radicalised in the UK over the last 10 years have viewed “an awful lot” of this graphic material.

Bilal Abdullah, a medical doctor, took part in the Glasgow terror attack in 2007 (Photo: Scotland Yard)

This range of different causes means there is no one simple solution, says Prof Silke, who urges a more “sophisticated” approach.

“We have to work with communities who feel isolated from mainstream society and feel that they are being treated as second class citizens,” he said.

One of the most important tools, he adds, is the Governments’s Channel Programme, which identifies individuals – particularly teenagers – who are at risk of radicalisation and works with them to try to divert them away from extremism.

“It is vital that the right resources are there in terms of social workers and police to make sure that work continues,” he said.